Free-cutting steels



June 11, 1968 Filed May 4. 1966 E. J. PALIWODA ET AL FREE-CUTTING STEELS 2 Sheets-Sheet ii INVENTORS.

EUGENE J. PALIWODA JAMES C. MQMANUS fheirATTORNEY Unite This invention relates to free-cutting steels. It is more particularly concerned with very high sulfur-containing steels of high machineability and good hot-workability.

It has been known for many years that additions of sulfur improve the machineability of steel. It is also known from US. Patent No. 2,484,231, issued to F. T. Kent on Oct. 11, 1949', that the machineability of high-sulfur steels is further improved if their carbon contents are kept below about .06%, and Kent teaches that such steels can be commercially produced by utilizing low carbon ferromanganese to make the necessary manganese addition. He discloses steels containing up to .400%, but preferably not more than 330% sulfur. Other investigators have also concluded that it is not advantageous to produce freecutting steels with sulfur contents greater than that set out by Kent. F.W. Boulger, H. A. Moorhead and T. M. Garvey in their paper, Superior Machineability of MX Explained, which appeared in The Iron Age for May 17, 1951, pp. 90-95, report that the beneficial effects of sulfur diminish or taper off rapidly above a content of about .25%. The addition of lead to high-sulfur steels brings about a further improvement in their machineability, but requires special addition techniques to bring about its uniform distribution in the ingot, and ventilating apparatus to remove the poisonous fumes produced by the volatilization of lead in molten steel.

It is an object, therefore, of our invention to provide very high sulfur-containing steels with machining properties superior to those heretofore attainable from lead-free steels. It is another object to provide such steels that have good hot-Working properties. It is another object to provide a process for manufacturing such steels. Other objects of our invention will appear in the course of the following description thereof.

We have found that steels having sulfur contents in excess of .4% up to at least .5 display machineability increased in proportion to their sulfur contents when their sulfur is in the form of globular sulfides rather than narrow elongated sulfides commonly denominated stringers. The exact composition of these globular sulfides is not known; however, We have found that they do not form in the absence of sufficient oxygen. To insure their formation, it is also necessary that the steel be substantially free of other elements which have a stronger tendency than sulfur to combine with oxygen.

The constituents normally present to which the above restriction applies are silicon and carbon. We prefer to limit the silicon content of our steels to a value of .01% and the carbon in a way to be described. When the silicon is no greater than .01%, we find that steels having sulfur contents in excess of .4% display proportionately improved machineability if the ratio of their oxygen to sulfur contents is greater than about .07. This means, of course, that a steel with better than .4% sulfur content must have better than about .028% available oxygen content. While this figure is appreciably higher than oxygen contents normally found in such steels, it is not too ditficult to achieve.

While manganese is a deoxidizer, we do not include it in the restriction above-mentioned as it appears to be a constituent of the desirable globular sulfides and is necessary in substantial amounts to render the steel hot-workable. However, the carbon and silicon contents of the ferrornanganese used for manganese additions must be States Patent such that the maximum levels of carbon and oxygen in the steel are not exceeded.

FIGURE 1 attached represents a photo micrograph taken at a magnification of 500 diameters of a steel of our invention identified as Heat V-1155. This steel contains .49% sulfur and 045% oxygen and has an oxygen to sulfur ratio of .088. The inclusions 1--1 which appear in the figure are sulfides and are seen to be globular in shape. The steel possessed very good machineability. For comparison FIGURE 2 represents a like photo-micrograph of steel from Heat V-ll99, which also contained .49% sulfur, but only .O20% oxygen. Its oxygen to sulfur ratio is .041, well below the lower limit of our invention. The sulfide inclusions 2-2 are seen to be elongated into stringers and the machineability of the heat was poor. The heats of steel above-mentioned are referred to hereinafter.

As might be expected, steels of the high sulfur and oxygen contents above-mentioned have pronounced tendencies to hot-shortness, and must be hot-worked with considerable care. We have further discovered that the hotworkability of these steels is markedly improved by decreasing their carbon contents. We have found empirically that these steels can be rolled successfully with only ordinary care if their carbon contents are adjusted so that the product of their carbon and oxygen contents, measured in percentages by weight, is less than about 10 X 10- This means, of course, that a steel with an oxygen content of .04% must have a carbon content not greater than about .025 While this figure is lower than is normally found in free-cutting steels, it is not so low as to be impracticable.

A process presently preferred by us for making the very high sulfur steels above-mentioned employs as its starting material a melt of ordinary low-carbon steel made under the oxidizing conditions that ordinarily exist in an open hearth furnace or a basic oxygen furnace. The oxygen content of this steel depends on its carbon content, being higher with lower carbon contents and vice versa. The steel is tapped into a ladle and the additional sulfur required to bring the sulfur content to the high values mentioned herein is added at this time. If phosphorus and manganese contents higher than those of conventional low-carbon steel are desired, these additions are also made at this time. We then vacuum degas the melt by any of the known techniques to lower its carbon level. When a substantial manganese addition is required, we prefer to add only part thereof to the steel before vacuum degassing and the remainder after vacuum degassing.

Our preferred starting material has the following composition: Percent C .03/.05 Mn .08/.3O P .01/.03 Si max .01

This composition is that of conventional low-carbon steel, which normally is made to a sulfur maximum of about .04%. This limitation, of course, is of no significance for our steel. The steel in the ladle after additions of sulfur, phosphorus, and manganese has the following composrtron. Percent C .03/.05 Mn .08/.60 P .05/.15 S .4/.S Si "max" .01

If desired, additional manganese is added after vacuum degassing to bring the manganese content of the steel to a maximum of about 1.20%.

The steel is degassed to a carbon level of .01% or lower. The corresponding oxygen content is about 07%.

We control the vacuum degassing treatment to produce a steel having the product of its carbon and its oxygen contents not greater than 10 10 After the steel is vacuum degassed and adjusted to provide the desired manganese content, it is teem-ed into ingot molds in the usual way and is allowed to solidify. The ingots are hot rolled to bars, which are normally cold drawn to finished size. Our steels display no appreciable mold reaction and in the form of ingot or rolled product contain substantially the same amounts of carbon and oxygen as are found at the conclusion of vacuum degassing.

FIGURES 3, 4, and 5 attached illustrate the influence of the carbon-oxygen product of high sulfur steels on their hotwo.rkability. FIGURE 5 illustrates a steel comprehended by our invention whereas FIGURES 3 and 4 illustrate steels which are not. FIGURE 3 represents a hot rolled bar 3 from H at V-1185 containing 036% carbon and 039% oxygen. Its carbon-oxygen product therefore is 14.0 and the figure shows that the steel broke up at 4-4 during rolling. FI URE 4 is of a hot rolled bar 5 from Heat V1186 containing 075% carbon and 017% oxygen. Its carbon-oxygen product is 12.8X10- and it is so hot-short as is shown by surface defects 6-6 that the rolled product is unuseable.

FIGURE 5 on the other hand is a hot rolled bar 7 from Head V-1155 of our invention containing 018% carbon and 043% oxygena carbon-oxygen product of 7.7 10 The bar is seen to have an excellent surface.

Where stcelmaking practices result in a melt of low oxygen content, we have found that that oxygen content is increased by the addition of oxygen-releasing compounds. By suitable additions of compounds such as sodium bi-sulfate, ammonium bi-sulfate and molybdenum tri-oxide, steels with sulfur contents in the range of .44% can be made to contain .04% to better than 05% oxygen as well.

The compositions and machineability indices of steels of our invention are listed in the appended Table I together with those of some prior art steels. It will be observed that the first six heats listed have especially good machineability as expressed in cutting speed measured in surface feet per minute. The machineability test employed for these determinations was designed specifically for free-cutting steels and is described in US. Patent No. 2,269,360, issued to A. A. Cruickshank on Jan. 6, 1942. The test measures rate of returning and does not provide information as to chip characteristics, power rerelatively small amounts of steel can be evaluated, but the test results correlate quite well with those obtained by the constant pressure lathe technique insofar as machineability effects of carbon, silicon and sulfur are concerned.

All machineability tests were made on 1 inch diameter round bars of the steel, cold drawn from hot rolled round bars. The normal draft was inch, from stock of 1% diameter, but it was found by experiment that Heats V1096 and V-1098 could be drafted inch without undue hardening and with a machineability results above set out.

Heats V-1185, V-1186, and V-1199 illustrate steels otherwise similar to the first six heats, but which do not conform to our invention. The first two could not be hot rolled and it was, therefore, not possible to determine their machineability. The oxygen-sulfur ratio of Heat V-1185 is the highest of the steels tabulated, and it would be expected to exhibit good machineability. However, its carbon-oxygen product is much too high for satisfactory hot rolling performance. Heat V-1186 likewise could not be hot rolled. Heat V-l199 rolled satisfactorily as would be indicated by its carbon-oxygen product of 5.0 10 but its index of machineability was only 100 s.f.m. which agrees with its low oxygen-sulfur ratio of .041.

The machining properties of our steels compare favorably with those of grade C-12Ll4, which is specified to contain .26 to sulfur and .15 to .35% lead. The average machineability index of steels of this grade commercially available is about 145 s.f.m. Lead-free grade C1213, which contains .24 to 33% sulfur, displays somewhat lower machineability.

The mechanical properties of the steels of our invention are tabulated in Table II and are there compared with those of commercially available free-cutting steel grades C-12Ll4 and C-1213. It will be observed that ultimate tensile and yield strengths of our steels in the hot rolled state are slightly lower than those of commercially available grades. The ultimate tensile strength and yield strength of our steel in cold finished bar form are, however, entirely comparable to those of steels commercially available, as are its elongation reduction of area.

The machineability of our steel described herein is further improved by the addition of lead. in conventional amounts.

TABLE I Heat 0 M11 P S Si N O OXC OIS Machineability X10 Index (s.f.m.)

.04 O77 007 010 03s 5. .086 .19 076 42 003 012 038 7. 22 090 160 16 078 45 O10 O14 033 6. 27 073 160 .06 .067 49 015 016 .043 7. 74 088 .05 068 .54 012 014 03E) 5. 46 072 150 .05 .109 48 019 014 040 6.00 083 150 98 075 40 007 014 039 14.00 .097 93 109 48 012 018 017 12. 80 035 98 086 49 012 014 020 5.00 041 100 TABLE II Ultra High C-l2L14 (J-1213 Sulfur Steels I. Hot Rolled Bar Product:

Ultimate Tensile Strength (p.s.l.) 58, 000/60, 800 50, 000/65, 000 55, 000/70, 000 0.2% Yield Strength (p.s.i.)- 33,800/41, 100 30, 000/ 15, 000 33, 000/45,000 Elongation (percent) 32. 0/36.5 30, 0/40.0 25. 0/30. 0 Reduction of Area (percent) 50. 8/55. 3 45. 0/55. 0 45. 0/55. D II. Gold Finished Bar Product:

Ultimate Tensile Strength (p.s.i.). 77, 300/86, 600 67, 000/82, 000 80, 000/05, 000 0.2% Yield Strength (p.s.i.) 64, 400/81, 600 60, 000/75, 000 75, 000/85, 000 Elongation (percent) 14. 5/20. 5 10. 0/20.0 10. 0/20. 0 Reduction of Area -14. 2/48. 9 25. 0/50. 0 40. 0/50. 0

quirements, or other aspects of machineability. Cutting tools of controlled moderate softness are used to turn screw stock at progressively increased speed. The highest speed at which two cubic inches of metal can be turned without exceeding a prescribed tool wear is termed the machineability index or rating. The principal ad- We claim:

1. Free-cutting steel containing up to about 05% carbon, about .080 to 1.20% manganese, sulfur exceeding .4%, about .05 to .15 phosphorus, silicon not exceeding about .01%, oxygen in amounts such that the oxygensulfur ratio is not less than .070, the balance being iron vantage of this special test is that the cutting quality of 75 and incidental impurities.

5 6 2. A readily hot-workable free-cutting steel of claim 1 is less than 10 1O and teeming the steel into ingot in which the product of oxygen and carbon contents is molds. less than 10 10 5. The process of claim 4 in which manganese is added 3. Free-cutting steel of claim 1 in which the sulfur conto the steel aflfif Vacuum degassing to bring its manga- [ent does not exceed about 5 nese content to a value not exceeding about 1.20%.

4. The process of making free-cutting steel comprising References Cited preparing under oxidizing conditions a melt of steel containing about .03 to .05% carbon, about .08 to .60% UNITED SFFATES PATENTS manganese, about .05 to .15% phosphorus, sulfur exceed- 10 1 g t 0 Y 0' 1 en 105. .4/0, silicon not exceedtng about .Olfiz, the balance 2,485,358 10/1949 Case 75 123 belng 11'01'1 and 1ncrdental impurities, vacuum degasslng the melt until the product of its carbon and oxygen contents CHARLES N. LOVELL, Primary Examiner. 

4. THE PROCESS OF MAKING FREE-CUTTING STEEL COMPRISING PREPARING UNDER OXIDIZING CONDITIONS A MELT OF STEEL CONTAINING ABOUT .03 TO .05% CARBON, ABOUT .08 TO .60% MANGANESE, ABOUT .05 TO .15% PHOSPHORUS, SULFUR EXCEEDING .4%, SILICON NOT EXCEEDING ABOUT .01%. THE BALANCE BEING IRON AND INCIDENTAL IMPURITIES, VACUUM DEGASSING THE MELT UNTIL THE PRODUCT OF ITS CARBON AND OXYGEN CONTENTS IS LESS THAN 10X10-**4, AND TEEMING THE STEEL INTO INGOT MOLDS. 